THERMOSTATIC EXPANSION VALVES Part 1

Refrigeration Service Engineers Society 1666 Rand Road Des Plaines, Illinois 60016

Revised by Loren Shuck, CMS

INTRODUCTION evaporator pressure control valve.” That is, it maintains a constant outlet pressure, regardless of The thermostatic expansion valve (frequently abbre- changes in inlet liquid pressure, in load, or in other viated TEV or, sometimes, TXV) has played a signifi- conditions. cant role in the progress of the HVAC/R industry and its technology. Its value is based on its refrigerant The automatic expansion valve was a decided im- control capability and its adaptability to the many and provement over the hand-operated expansion valve. varied applications of the refrigeration cycle. It maintained temperature more evenly, and controlled the frost line of the evaporator better. Further- The development of many system components has more, it closed the liquid line when the compressor been a by-product of technical evolution. This applies stopped, and prevented excessive flooding when the to the thermostatic expansion valve. In the early days compressor started. of mechanical refrigeration, a hand-operated needle valve was used for refrigerant control. This device However, this device also had disadvantages and gave some measure of control in applications where limitations. It tended to overfeed refrigerant to the the load was constant. However, it could not respond evaporator when the heat load was low, as at the end to other conditions affecting the amount of refrigerant of the run. It also tended to starve the evaporator passing through it—for example, changes in liquid when the heat load was high, as at the start of the pressure caused by variations in compressor head run. Thus, its pull-down of temperature was slow. It pressure. The manual expansion valve required con- did not take advantage of the full area and capacity stant supervision in locations where a varying load of the evaporator at the start of the refrigeration could produce starved evaporator or liquid refrigerant cycle. floodback conditions. In the late 1920s, another device—the thermostatic The introduction of “float”-type metering valves was a expansion valve—was introduced. It overcame the step toward automatic refrigerant flow. One type was limitations of both the hand-operated expansion designed to control refrigerant in the low side, and valve and the automatic expansion valve. Figure 1 on another in the high side. The float-type valve oper- the next page shows a typical example of a modern ated in the same manner as a boiler water-level con- thermostatic expansion valve (sometimes also called trol to maintain a flooded evaporator. It was highly a thermal expansion valve). The purpose of the TEV efficient in serving that purpose—however, it could is to control the flow of refrigerant to an evaporator. not be applied to a dry or direct-expansion evaporator system, which was gaining favor. Even so, the If the evaporator were to remain active through all float-type valve did not disappear from the field. It is variations in the heat load, it would contain only liquid still used in industrial process refrigeration systems. and saturated vapor—no superheated vapor. In prac- It is also used in large water chillers, especially those tice, this is not a desirable condition. Some of the with centrifugal compressors. evaporator must be used to superheat the vapor, or else there will be liquid refrigerant in the suction line, Efforts to overcome these inadequacies produced which can slug the compressor. The TEV is actually what is called the “automatic expansion valve.” A a constant superheat valve—a more descriptive and more accurate description would be a “constant accurate name, though rarely used.

This document is a revision of prior publication 620-40. A N Y This chapter covers the theory of operation, proper selection, and application of thermostatic expansion

C O M P valves. The control of superheat is one of the basic

V E functions of thermostatic expansion valves. The dis- A L V cussion that follows will better acquaint you with the principle of superheat.

S P O R L A N TEMPERATURE-PRESSURE RELATIONSHIPS

If you are familiar with the concept of temperature- pressure relationships already, then you should have little difficulty understanding the basic function of a thermostatic expansion valve. Figure 2 below illustrates the changes that take place in 1 lb of water at atmospheric pressure.

ª Sensible heat is added. It raises the temperature from 32°F to 212°F (from the freezing point to the boiling point). Figure 1. Typical thermostatic ª Heat (sensible heat) is continually added until the expansion valve (TEV) temperature reaches 212°F. At this point, the water must absorb another 970.4 Btu/lb (latent heat of vaporization) to change completely to The name of this device can give the impression that steam at 212°F. it directly controls temperature. Some service technicians mistakenly have tried to adjust a thermostatic ª Additional sensible heat raises the temperature expansion valve to control refrigerator temperature. of the steam above 212°F.

Superheated vapor

20°F superheat (232°F – 212°F = 20°F) 232

Saturated steam @ 14.7 psia 212 ° F , Boiling point Point of complete vaporization e m p r a t u T

Latent heat of vaporization = 970.4 Btu 32

200 400 600 800 1000 1200 Enthalpy, Btu/lb

Figure 2. Water at atmospheric pressure

2 The temperature above 212°F is called 68 70 superheat. It is still measured in degrees. 60 Figure 2 shows an example. The tempera- Pressure, Temperature, ture of the steam at atmospheric pressure psig °F is 232°F, which means that it has been 50 68 40 Curve shows refrigerant superheated 20°F (232°F – 212°F = 20°F). 28 5 40

Pressure, psig pressure-temperature 9 –22 characteristics The principles are the same when you ana- 28 30 lyze the effect of superheat on a refrigerant. Figure 3 shows the effect of superheat 20 on 1 lb of liquid R-22. At atmospheric pressure, the addition of heat makes it boil at 9 10 –41°F. After all the refrigerant vaporizes, –22 any added heat will raise its temperature –40 –20 5 10 20 40 above –41°F, or superheat it. Note that it Temperature, °F takes 100 Btu (latent heat of vaporization) to vaporize the entire pound of liquid R-22. Adding more heat will superheat the vapor. Figure 4. Boiling points of R-22 at various pressures If the temperature is raised to –21°F, the vapor has been superheated 20°F. Figure 4 above shows the boiling points of R-22 at Figure 3 is only an example of the principle of super- various pressures. Note the curved line. It shows how heat. In fact, service technicians do not work with any the saturation or boiling temperature changes as refrigerant at atmospheric pressure. You already pressure changes. As the pressure increases, it know that reducing pressure on a liquid lowers its takes a higher temperature for the refrigerant to boil. boiling point. It then takes less heat to vaporize it. The curved line also represents the R-22 pressure- Conversely, increasing the pressure on a liquid raises temperature characteristics. The horizontal line the boiling temperature. shows the temperature in degrees Fahrenheit. The

20 Superheated vapor 10

° F –10 20 F superheat [(–21 F) – (–41 F) = 20 F] –20 ° ° ° ° Boiling point Point of complete –30 vaporization Temperature, Saturated R-22 @ 14.7 psia –40

–50 Latent heat of vaporization = 100 Btu

–60 0* 10 20 30 40 50 60 70 80 90 100 Enthalpy, Btu/lb

*Based on 0 Btu for the saturated liquid at –40°F

Figure 3. R-22 at atmospheric pressure

3 vertical line shows pressure in psig. The dotted lines tubing wall, it causes the refrigerant in the coil to heat in Figure 4 show some of the coordinates on the up and boil. Assume that the refrigerant flow is at a curve at which the refrigerant will boil. relatively slow rate. If this is the case, all the liquid boils into vapor close to the entry point. You can see Follow this relationship up and down the pressure- an example of this at point A in Figure 5, where the temperature scale in Figure 4. You can determine a temperature is 20°F. series of boiling points—for example, 40°F at 68 psig, 5°F at 28 psig, –22°F at 9 psig, etc. The curve itself From this point on, the boiling vapor picks up super- is simply a line drawn through all of these points. heat. The farther the vapor travels and the longer it is Other refrigerants have different curves. No two exposed to heat, the greater the superheat becomes. refrigerants have identical pressure-temperature At point B, the vapor’s temperature is 30°F with 10°F characteristics. superheat. At the fifth return bend, the temperature is 40°F with 20°F superheat. THE EFFECTS OF SUPERHEAT ON A REFRIGERATION SYSTEM Now look at point C. The difference between vapor temperature (50°F) and boiling temperature (20°F) Now that the basic principle of superheat has been equals 30°F of superheat. At this point, the suction covered, let’s relate the various factors to a simple pressure of the compressor is 43 psig. refrigeration system. Assume that the system consists of a bare tube evaporator, compressor, condenser, Opening the hand expansion valve farther increases and receiver. It also has the most basic of refrigerant the flow of refrigerant. Figure 6 shows the result. The control devices, the hand-operated expansion valve. increased flow rate means that the refrigerant must Figure 5 shows such a basic system. reach point B before complete vaporization takes place. This reduces the evaporator coil surface that is Opening the hand expansion valve slightly allows a available for building up superheat. As a result, there small amount of refrigerant to enter the exposed coil. is only 20°F superheat at point C. Higher suction Air around the coil is at a higher temperature than the pressure results from the greater load placed on the refrigerant. When the heat from the air penetrates the compressor by the additional refrigerant.

Liquid line

Hand expansion valve Condenser 20°F Receiver 20°F Point of complete 43 psig A vaporization

30°F Head 10°F superheat B pressure 40°F 20°F superheat

Compressor 43 psig suction pressure 50°F gas temperature 30°F superheat

Figure 5. Simple refrigeration system

4 Liquid line

Hand expansion valve Condenser 21°F Receiver 21°F 44 psig A 21°F Point of complete Head B vaporization pressure 31°F 10°F superheat

Compressor 44 psig suction pressure 41°F gas temperature 20°F superheat

Figure 6. Additional flow reduces superheat

Now look at Figure 7. The hand expansion valve is damage to the compressor. The compressor is open even farther. So much refrigerant flows that it designed to compress a gas, not a liquid. cannot completely vaporize in the evaporator. This causes liquid to flood back to the compressor. The These examples demonstrate the ideal evaporator result is wasted refrigerant and greatly reduced com- condition. The goal is to keep the point of complete pressor capacity. The greatest concern is possible vaporization at the outlet of the evaporator. Note in

Liquid line

Hand expansion valve Condenser 25°F Receiver 25°F 49 psig A 25°F Head pressure B Flood back 25°F

Compressor 49 psig suction pressure 25°F gas temperature 0°F superheat

Figure 7. Too much flow produces floodback

5 Liquid line

Hand expansion valve Condenser 24°F Receiver 24°F 47.6 psig A

24°F Head Point of pressure complete B vaporization 24°F

V 29°F C 5°F superheat

Compressor 47.6 psig suction pressure 34°F gas temperature 10°F superheat

Figure 8. Proper flow results in correct superheat

Figure 8 that superheat is 0°F at point V. Ideally, the to open the valve. The addition of several degrees entire surface of the evaporator performs its refriger- produces a steady, rated flow of refrigerant. ation function. Only dry, superheated vapor reaches the compressor. Figure 9 shows the level of efficiency achieved with the TEV.You have seen that low superheat at the out- In practice, it is impossible to obtain this condition. let of the evaporator is desired. Thus, you place the However, the TEV allows you to reach a level close to TEV bulb at this point to measure this condition. In the ideal. It takes only several degrees of superheat the proper position, its temperature will be about equal to suction gas temperature.

Thermostatic Before examining TEV operation in expansion valve Refrigerant saturation depth, first review the following terms. temperature They are closely associated with valve operation:

Liquid line Liquid line. The piping that carries liquid refrigerant from the condenser to the expansion valve. Remote bulb Refrigerant saturation temperature. The temperature at which refrigerant boils in the evaporator. Point of complete vaporization Superheat Suction gas Complete vaporization. The point at temperature which liquid refrigerant is entirely changed to vapor, governed by the Figure 9. TEV installed at entrance to evaporator amount of refrigerant in the evaporator.

6 SPORLAN Diaphragm case Superheat. The heat added to vapor beyond the Capillary point of complete vaporization. tube V LECOMP ALVE Suction gas temperature. The temperature of refrigerant vapor measured along the suction Push line between the evaporator and the compressor. rods ANY

THERMOSTATIC EXPANSION VALVE OPERATION

Principal parts Seat Figure 10 shows the essential parts of a typical thermostatic expansion valve—the diaphragm Pin carrier case, capillary tube, bulb, seat, pin carrier, spring, spring guide, adjusting stem packing, Spring Bulb adjusting stem, push rods, and inlet strainer. Spring guide Note that there is no direct mechanical linkage Inlet strainer between the adjustment stem and the pin. The Adjusting adjusting stem serves only to vary the spring stem pressure. packing

The diaphragm, inside its case, reacts to bulb Adjusting pressure. It transmits its motion through the push stem rods to the top of the pin carrier. When the pin moves away from the seat, liquid refrigerant flows into the evaporator. The pin carrier is long enough to ensure straight-line movement and Figure 10. Essential parts of a TEV precise pin-seat contact. As noted, the adjusting stem serves only to control spring pressure against the pin carrier.

Operation

From the positioning of its parts, it may seem like the ➀ principle of TEV operation is bulb pressure opposing Bulb pressure spring pressure. But this is not the case. There are three fundamental pressures that affect the opening and closing of this control device: ➁ Bulb temperature = Evaporator pressure ª saturation temperature bulb pressure + superheat

ª spring pressure

ª evaporator pressure. ➂ Figure 11 shows the application of these three fun- Spring pressure damental pressures. A thorough knowledge of the relationship among these three pressures is needed Figure 11. TEV operation

7 Point of complete vaporization to determine the final pressure—that of the bulb. Bulb pressure is applied through the capillary tube to the top of the valve diaphragm. This pressure opposes the sum of the evaporator pressure and the spring Refrigerant saturation pressure. It acts to open the valve. When a system is temperature operating, the bulb charge is part liquid refrigerant Diaphragm and part saturated vapor. Bulb temperature equals suction gas temperature. Suction gas temperature is higher than refrigerant saturation temperature. The Evaporator pressure increase is produced by superheat.

The higher bulb temperature applies more pressure to the top of the diaphragm than the opposing evaporator pressure. However, opening the valve requires enough pressure difference (increased superheat) to overcome the additional effect of spring pressure. Figure 14 provides a closer look at how the bulb Figure 12. Evaporator pressure relates to pressure acts on the top of the diaphragm. refrigerant saturation temperature When the bulb and system are charged with the same refrigerant, both the evaporator pressure under for complete understanding of valve operation. As the diaphragm and the bulb pressure on top follow shown in Figure 11 on the previous page, in order for the same saturation curve. the valve to open, bulb pressure (1) must equal the sum of the evaporator pressure (2) and the spring Consider the results of this relationship. Pressure on pressure (3). the top of the diaphragm is induced by bulb saturation temperature. Pressure on the bottom is induced Evaporator pressure is a key factor in the operation of a thermostatic expansion valve. Unfortunately, it is often overlooked in an analysis of valve operation Superheat problems. Evaporator pressure is applied under the Bulb diaphragm in the direction of closing the valve, as shown in Figure 12 above. It is important because of its relationship to refrigerant saturation temperature. Point of complete Suction gas vaporization Evaporator pressure always equals the pressure cor- temperature responding to refrigerant saturation temperature. Look back at Figure 4 for a moment. It shows examples of this pressure-temperature relationship. One example is that a saturation temperature of 40°F for Bulb R-22 produces an evaporator pressure of 68 psig. pressure

In addition, the spring transmits pressure through the top of the pin carrier and the push rods. It is also applied to the underside of the diaphragm. Thus, spring pressure further implements closing of the valve.

Figure 13 shows more of the evaporator. It shows the points at which superheat and suction gas temperatures can be measured. These temperatures are used Figure 13. Bulb pressure acts to open valve

8 by evaporator saturation temperature. They become Superheat equal and opposing, canceling each other out. You can see, then, that pressures induced by refrigerant Bulb saturation temperature exert equal force. They act on both sides of the diaphragm. This leaves superheat and spring pressure the two factors that regulate the valve. These two opposing factors maintain a delicate pressure balance on both sides of the valve diaphragm. This permits the valve to operate over a Refrigerant saturation temperature wide range of evaporator pressures. It can handle Bulb pressure light as well as heavy loads on the evaporator. The thermostatic expansion valve is, in effect, a superheat regulator. Evaporator pressure

Occasionally you may hear a service technician say, “I opened the expansion valve” or “I closed the expansion valve.” Actually, the adjustment was simply a change in spring pressure. Increased spring pressure requires more pressure induced by superheat to Spring pressure oppose it. The valve throttles back. On the other hand, reduced spring pressure requires less pressure induced by superheat to oppose it. The valve Figure 14. Bulb pressure and the effect of opens. superheat

For any spring setting, when the load on the evaporator increases, the valve will underfeed the evaporator. As a result, suction pressure will drop or bulb temperature will increase—or both conditions may exist. In any case, the valve will open wider. If the evaporator load is Bulb pressure 64 psi Converted to temperature 37°F reduced, the valve will overfeed the evaporator. As a result, suction pressure will increase or bulb pressure will decrease, or Diaphragm both. Existing spring pressure will throttle Evaporator inlet pressure the valve to the closed position. 52 psi

Figure 15 illustrates conditions in a typical system, and shows a closer relationship between theory and practice. Evaporator 52 psi pressure is 52 psig, and spring pressure is 12 psig. They exert a combined closing Spring pressure 52 psi 37°F force of 64 psig. Assume that the bulb is 12 psi charged with the same refrigerant as the system. A bulb pressure of 64 psig will be required to equalize pressure on both sides Evaporator of the diaphragm. At 28°F refrigerant satu- outlet pressure ration temperature, a superheat of 9°F is 52 psi required to increase suction gas and bulb temperature to 37°F. This would produce Figure 15. Superheat equals bulb temperature minus the required 64 psig in the bulb. saturated evaporator temperature

9 In a system idle for a long period of time, pressure in diaphragm. When this pressure can overcome the the bulb and above the diaphragm will be almost combined evaporator pressure and spring tension enough to overcome spring pressure and open the below the diaphragm, the valve opens. This restores valve. Compressor start-up will immediately lower the or increases refrigerant flow into the evaporator. refrigerant pressure under the diaphragm. The bulb pressure will then be enough to oppose the spring When the system’s pressure or temperature control pressure. It will force the diaphragm down and open stops the compressor, evaporator pressure immedi- the valve. ately increases. This is because it is no longer subject to the compressor suction effect. It will increase Liquid refrigerant will then flow through the open until the combined evaporator pressure and spring valve as a spray. It will fill the evaporator with wet and tension pressure can overcome vapor pressure in the saturated vapor. If all of the liquid refrigerant evapo- bulb and above the diaphragm. When it does, the rates and becomes superheated vapor before it diaphragm raises and the TEV closes. reaches the suction line, the thermal expansion valve remains open. PRESSURE DROP ACROSS THE EVAPORATOR

The evaporator continues to absorb heat. The sur- This discussion of TEV operation has not yet consid- rounding temperature will decrease until wet and sat- ered pressure drop through the evaporator. It was urated vapor extends to the point in the suction line omitted for simplification. In actual system operation, where the capillary tube bulb is clamped. When this however, pressure drop will exist and is a factor. To occurs, the reduced degree of superheat cools the overcome the effect of pressure drop, inlet pressure suction line and bulb. As a result, vapor pressure in is used to operate the valve. the bulb and above the diaphragm decreases. This permits spring pressure to close the valve, either Inlet evaporator pressure acts on the underside of wholly or partially. the diaphragm. It feeds through a passageway from the low-pressure side of the valve. This passageway Refrigerant flow is now stopped or reduced. It is called an “internal equalizer,” shown in Figure 16. remains so until superheat at the bulb location (A thermostatic expansion valve also may have an increases vapor pressure in the bulb and above the external equalizer connection, which will be covered in greater detail later in this chapter.)

By now, you should be able to form one firm conclu- sion. It is that the function of a thermostatic valve is to keep the refrigerating capability of the evaporator at the highest level possible under all load conditions. It is not to be confused with a suction pressure control device. Nor can it, in any way, control compressor running time. Internal equalizer REMOTE BULB CHARGES

The refrigeration service technician must know the correct replacement for an inoperative TEV.You may face a situation where someone else has incorrectly Push rods selected the existing valve. A major factor in valve replacement is the type of remote bulb charge.

For correct valve replacement, you must be familiar with refrigerant charges in the remote bulb and tubing Figure 16. TEV with internal equalizer of TEVs. There are three general types:

10 The gas charge

A TEV with a gas-charged remote bulb contains the Bulb colder Bulb warmer same refrigerant as that in the system. However, it uses a limited amount of liquid refrigerant. All of the liquid in the bulb will evaporate at a certain remote bulb temperature. The charge becomes a saturated Control always in bulb vapor. Further increases in remote bulb temperature will not cause any substantial increase in pressure. Figure 17. Bulb maintains control of power Such temperature increases will only superheat the element in liquid-charged TEV vapor.

Thus, in a gas-charged valve, maximum pressure on ª the liquid charge, in which the refrigerant used in the upper surface of the diaphragm is limited by the the remote bulb and tubing is the same as the amount of charge in the remote bulb. There is an refrigerant being controlled by the valve equation that describes this relationship, which is shown in Figure 19 on the next page. Remote bulb ª the gas charge, a limited liquid charge pressure (P1) is equal to evaporator pressure (P2) plus spring pressure (P3), or P1 = P2 + P3. ª the cross charge, in which the refrigerant used in the remote bulb and tubing is different from the This equation shows that if remote bulb pressure can refrigerant being controlled by the valve. be limited to a certain level while spring pressure remains constant, evaporator pressure can be limited The liquid charge to a certain level and no higher. When remote bulb pressure is limited, evaporator pressure is also lim- Figure 17 shows a conventional liquid-charged ited. The point at which all liquid in the remote bulb is remote bulb and tube element. As you can see, the evaporated is the maximum operating pressure bulb is large enough and contains enough liquid so (MOP) of the valve. Look at the example shown in that the control point is always in the bulb. Some upper refrigerant remains liquid regardless of bulb temperature. The temperature of the diaphragm case or capillary tubing may get colder than the bulb tem- ) re u perature. Some of the vapor may condense in either s s re of them. However, there will always be enough liquid p r to refrigerant in the bulb to ensure control at that point. ra o p This becomes very important in low-temperature a v e applications. + re su ) s re e u r s Look at Figure 18. Note the two curves, one on top p s g re rin p of the other. You can see that, with the same refrig- Pressure p (s lb e bu rc ( erant in both the bulb and the evaporator, pressure- fo e g rc temperature relationships are identical. Now apply sin fo lo ng C ni these curves to system operation. They show that pe there is always a direct relationship, across the O diaphragm, of bulb pressure changes to evaporator pressure changes. Thermostatic expansion valves with liquid remote bulb charges normally are used when there is a very narrow or limited evaporator Temperature temperature range. This is a condition outside the scope of other remote bulb charges. Figure 18. Conventional liquid charge

11 P1 Bulb pressure the evaporator, but it will not produce enough pressure in the remote bulb to open the valve.

Evaporator pressure will now go down. It will drop below the MOP point of 100 psig. Only then can the P2 remote bulb again control the valve and feed refrig- Evaporator pressure erant into the evaporator. Thus, a gas-charged TEV also provides positive motor overload protection on some systems. This is because of the limiting effect of the maximum operating suction pressure. It also prevents possible floodback on start-up.

To lower maximum operating pressure, increase the P3 superheat setting (increase spring pressure). To raise Spring pressure it, decrease the superheat setting (decrease spring pressure). This adjustment works because spring Figure 19. Evaporator pressure is limited by pressure, together with evaporator pressure, acts bulb pressure limit directly on the remote bulb pressure through the diaphragm.

Figure 20. Assume that the system pictured has an You should always install a gas-charged valve in a R-22 gas-charged expansion valve. It has a 100-psig location warmer than that of the remote bulb. The maximum operating pressure and a 10°F superheat reason for this is that a gas-charged power element setting. Now, assume that the evaporator pressure contains liquid up to the MOP point. It is very impor- increases to 100 psig. The total force under the tant for this liquid to remain in the remote bulb, as diaphragm will be 100 psig plus 11.5 psig (the spring shown in Figure 21. The remote bulb tubing must pressure setting for 10°F superheat). This equals never contact a surface colder than the remote bulb. 111.5 psig. Remember that the charge in the gas- If it does, the charge will condense at the coldest charged remote bulb and tube is limited. When the point and the valve will not function properly. remote bulb reaches the saturation temperature that corresponds to 111.5 psig, the entire charge has Generally, the application of gas-charged thermosta- evaporated. Only vapor exists in the bulb. There will tic expansion valves is not limited. However, they be some added superheat of refrigerant gas leaving work most efficiently in such applications as water

111.5 psig maximum

Liquid is completely Evaporator pressure evaporated 100 psig at 65°F and Spring pressure 111.5 psig 11.5 psig

Evaporator pressure (also suction pressure) must be less than 100 psig before valve pin will open, regardless of remote bulb temperature

Figure 20. System with 100-psig maximum operating pressure

12 chillers and air conditioning units that oper- P1 ate in a 30°F to 50°F evaporator temperature range. Bulb Remote bulb loses The cross charge control if charge condenses at P2 these points With a cross-charged remote bulb, the pressure-temperature curve of the refrigerant in P3 the bulb crosses that of the refrigerant in the system.

When the system evaporator temperature ranges from +50°F to –100°F, the TEV in use Drop of liquid must be in will incorporate one of the cross charges remote bulb for proper control available. This type of system often operates on what is termed a repetitive pull-down Figure 21. Control must remain in gas-charged cycle. In this cycle, the system starts up at a remote bulb high suction pressure. It shuts off when the pull-down reaches a much lower pressure. At the cutoff point, the compressor stops. Then the evaporator and remote bulb begin to warm Vapor cross charge. Research by TEV manufactur- up. The evaporator pressure rises more rapidly than ers has led to the development of a vapor-charged the remote bulb pressure. As a result, the TEV closes remote bulb. A vapor cross charge is designed for quickly. It stays closed even though the bulb temper- systems that operate with evaporator temperatures ature may rise to a higher level than the evaporator ranging from +50°F to –40°F. The vapor cross- temperature. charged bulb meets all the requirements for liquid- charged and gas-charged bulbs. And it meets the When high suction pressure exists, it takes high requirements for several of the liquid cross-charged superheat to produce enough bulb pressure to over- bulbs available. A valve with this type of cross charge come the closing pressure. The evaporator and bulb can replace a valve with any other type of charge if pressure curves are farther apart at high pressures. the system operates in the evaporator temperature As a result, valve opening requires higher than normal range noted. However, it is usually necessary to superheat. This is a desirable condition. It prevents readjust the superheat setting of the vapor cross- flooding and helps limit the compressor load. charged valve.

The difference between evaporator temperatures Surrounding temperature is not important to the loca- and bulb temperatures create the operating super- tion of a vapor cross-charged valve. This is because heats of the cross-charged TEV. It is apparent, then, it cannot lose control if the valve is colder than the that a cross-charged valve must be adjusted for the remote bulb. best superheat setting at the lowest evaporator temperature of the system in order to prevent floodback You also can use a vapor cross-charged valve with a of liquid refrigerant to the compressor. compressor, which requires motor overload protection. It is also available without MOP, when motor Liquid cross charge. Cross charges may be either overload protection is not required. When there is no liquid or vapor. There are three types of liquid cross requirement for starting or overload protection, use charges. One is designed for use in the commercial the non pressure-limiting type. This may be the case temperature range (from 40°F to 0°F). Another is for with smaller commercial units or window air condi- the low temperature range (from 0°F to –40°F). The tioners. When maximum operating pressure is third is designed for the ultra-low temperature range required, you must consider correct MOP in selecting (from –40°F to –100°F). a replacement valve.

13 16° SH ° F Liquid charge 9° SH 1. Maintains control under cross-ambient conditions 4° SH 2. Tends to allow floodback on start-up Superheat, 3. MOP is not available Normal range –40 050 Evaporator temperature, °F

16° SH ° F Gas charge

6° SH 1. Loses control under cross-ambient conditions 5° SH 2. Tends to prevent floodback on start-up

Superheat, 3. MOP is available Normal range –40 0530 0 Evaporator temperature, °F

20° SH

° F Liquid cross-charge

6° SH 6° SH 1. Maintains control under cross-ambient conditions 2. Tends to prevent floodback on start-up

Superheat, 3. MOP is not available Normal range –40 040 Evaporator temperature, °F ° F All-purpose vapor cross-charge

6° SH 6° SH 1. Maintains control under cross-ambient conditions 2. Tends to prevent floodback on start-up Superheat, 3. MOP is available Normal range –40 050 Evaporator temperature, °F

Figure 22. Comparing the types of remote bulb charges

14 Comparing the types of remote bulb charges ª undersized evaporator tubing

Each of the four remote bulb charges discussed—the ª extreme length of tubing per pass liquid charge, the gas charge, the liquid cross charge, and the vapor cross charge—has its own superheat ª improper joints characteristics. The curves in Figure 22 show the operating limits of each type based on a superheat of ª restrictive return bends 6°F. ª distribution headers of various types. These curves may not reflect true values. They are presented here simply to show the general advan- Certain pressure drops through the evaporator are of tages and disadvantages of each. From Figure 22, consequence: you can see why the vapor cross charge is a major development. It has the potential to maintain almost ª above 3.0 psi in the air conditioning range constant superheat. This makes it possible to get maximum refrigeration effect from an evaporator with ª 2.0 psi in the commercial range temperatures ranging from +50°F to –40°F. You can expect consistent superheat adjustments for the full ª 0.75 psi in the low-temperature or food freezer range of evaporator operation. This eliminates con- range. cern for floodback or starving of the evaporator at the extreme ends of the temperature range. These pressure drops will hold a TEV in a relatively “restricted” position. This will reduce system capacity THE EXTERNAL EQUALIZER unless a valve with an external equalizer is used. The evaporator should be designed or selected for the Look back at Figure 16, which shows a TEV with an operating conditions. The valve should be selected internal equalizer. Pressure at the valve outlet to the and applied accordingly. evaporator is applied through the bottom of the diaphragm. It is applied through a hole drilled in the The valve in Figure 24 on the next page has an inter- valve or through the space around the valve push nal equalizer. Pressure at the valve outlet is on the rods. Now look at Figure 23. It shows a TEV with an exter nal equaliz er. Note that packing Valve with external equalizer around the push rods keeps valve outlet pressure away from the evaporator side of the diaphragm.

External Instead, suction pressure is applied to the equalizer Push rod evaporator side of the diaphragm through tub- fitting ing. It is either soldered into an opening in the packing suction line near the evaporator outlet or into a reducing tee located there. This tubing is Evaporator connected to the external equalizer fitting on outlet pressure the valve, as shown in Figure 23. The pre- ferred tubing location in the suction line is downstream from the bulb. Push rods

Pressure drop between the valve outlet and Valve the evaporator outlet is the most important of outlet pressure the low-side pressure losses that affect valve performance. Sizable pressure drops can be caused in the low side of systems by: Figure 23. TEV with external equalizer

15 bottom of the diaphragm. It is also assumed that the of evaporator surface available for the absorption of evaporator shown has a negligible pressure drop. latent heat of vaporization of the refrigerant is Any large pressure drop through the evaporator will reduced. The evaporator is said to be “starved.” cause the valve to restrict refrigerant flow unless a valve with an external equalizer is used. The thermostatic expansion valve is being held in a somewhat “restricted” position. The pressure under- To understand this, look at Figure 25, which shows neath the diaphragm (at the evaporator inlet) is an example of an evaporator fed by a TEV with an higher than the pressure at the remote bulb, point C. internal equalizer. As you can see, there is a sizable Remember that the pressure drop across the evapo- pressure drop of 16 psig. rator, which causes this condition, increases with the load. Thus, the “restricting” or “starving” effect is The pressure at point C is 52.5 psig. This is 16 psig increased when the demand on the expansion valve lower than at the valve outlet, point A. The 68.5 psig capacity is greatest. at point A is the pressure acting on the lower side of the diaphragm. The valve spring is set at a compres- USING AN EXTERNAL EQUALIZER sion equal to a pressure of 15.5 psig. Thus, the required pressure above the diaphragm is 68.5 psig The external equalizer type of valve must be used to plus 15.5 psig, or 84 psig. This pressure corresponds compensate for an excessive pressure drop through to a saturation temperature of 50°F. Obviously, the an evaporator. The equalizer line can be connected in refrigerant temperature at point C must be 50°F for one of two ways: the valve to be in equilibrium. ª It can connect into the evaporator at a point But the pressure at this point is only 52.5 psig. The beyond the greatest pressure drop. corresponding saturation temperature is 28°F. As a result, a superheat of 22°F (50°F – 28°F = 22°F) is ª It can connect into the suction line at a point on required to open the valve. An increase in superheat the compressor side of the remote bulb. from 10°F to 22°F is needed. More of the evaporator surface must be used to produce this higher super- As a general rule of thumb, the equalizer line should heated refrigerant gas. This means that the amount be connected to the suction line at the evaporator

P1 = 84 psig

P2 = 68.5 psig

A 68.5 psig = 40°F

P3 = 15.5 psig

68.5 psig = 40°F

84 psig = 50°F C 68.5 psig and 50°F

Figure 24. TEV with internal equalizer on evaporator with no pressure drop

16 P1 = 84 psig

P2 = 68.5 psig

A 68.5 psig = 40°F

P3 = 15.5 psig

52.5 psig = 28°F B

84 psig = 50°F 52.5 psig and 50°F C

Figure 25. TEV with internal equalizer on evaporator with 16-psig pressure drop

outlet. When this arrangement is used, true evapora- Look again at the system shown in Figure 25. Now tor outlet pressure is imposed under the valve assume that there are the same pressure conditions diaphragm. Pressure drop through the evaporator in a system using a valve with the external equalizer does not affect operating pressures on the diaphragm. feature, as shown below in Figure 26. The same The thermostatic expansion valve will respond to pressure drop still exists through the evaporator. the superheat of the refrigerant gas leaving the Pressure under the diaphragm is now the same as evaporator. the pressure at the end of the evaporator (point C, or

P1 = 68.5 psig

P2 = 52.5 psig

A 68.5 psig = 40°F

P3 = 16 psig

B 52.5 psig = 28°F

68.5 psig = 40°F C 52.5 psig and 40°F

Figure 26. TEV with external equalizer on evaporator with 16-psig pressure drop

17 52.5 psig). The pressure required above the dia- ª 2.0 psi in the commercial temperature range phragm for equilibrium is 52.5 psig plus 16 psig, or 68.5 psig. (Note the increase in the spring pressure— ª over 0.75 psi in the low freezer temperature from 15.5 psig to 16 psig—needed to bring the TEV range. into equilibrium.) The 68.5-psig pressure corresponds to a saturation temperature of 40°F. Therefore, the The recommendations in Table 1 adequately cover superheat required is now 40°F – 28°F, or 12°F. The the requirements for most field-installed systems. In use of an external equalizer has reduced the super- general, install an externally equalized valve when heat from 22°F to 12°F.As you can see, the operating pressure drop between the evaporator inlet and the superheat has changed from 10°F to 12°F.This is the suction-line bulb location exceeds the maximums result of the change in the pressure-temperature listed in Table 1. characteristics of R-22 at the lower suction pressure of 52.5 psig. DUAL-DIAPHRAGM THERMOSTATIC EXPANSION VALVES However, in an operating system, where more evaporator surface is made available for boiling the liquid So far, this chapter has assumed that both sides of refrigerant, the evaporator outlet pressure will an expansion valve’s diaphragm are of equal area. increase. It will be above 52.5 psig. Thus, you can This is the case when the valve has a single see the advantage of using a TEV with an external diaphragm. However, there are valves with two equalizer, as opposed to one with an internal equal- diaphragms. One is subject to pressure developed in izer. Capacity will increase in a system that has an the remote valve, the other is subject to pressure evaporator with a sizable pressure drop. developed in the evaporator and by the adjusting spring. The two diaphragms can be of different sizes, A thermostatic expansion valve must be equipped as shown in Figure 27. with the external equalizer feature in order to provide the best performance in the following cases: There is an advantage in using two diaphragms, one acted upon by evaporator pressure and the other by ª when the pressure drop through an evaporator is remote bulb pressure. By proportioning their size, a in excess of the limits previously defined valve can operate at the same superheat at 5°F as at 25°F. Constant superheat can be maintained. This is ª when a pressure drop type of refrigerant not possible with a single-diaphragm valve. distributor is used at the evaporator inlet.

External equalizer application Evaporating temperature, °F The level of evaporator pressure drop that an internally equalized valve can cope with is 40 20 0 –20 –40 determined by two factors—the evaporator temperature and the refrigerant used. Use the Refrigerant Pressure drop, psi recommendations of valve manufacturers as R-12 guidelines. Generally speaking, a valve with 2.0 1.5 1.0 0.75 0.5 an external equalizer is required when an R-500 evaporator is subject to a pressure drop of any consequence. With R-22, for example, an R-22 externally equalized valve is recommended R-717 3.0 2.0 1.5 1.0 0.75 when pressure drop exceeds the following R-502 levels:

ª 3.0 psi in the comfort air conditioning Table 1. Greater evaporator pressure drop requires evaporator temperature range external equalizer

18 Power element bulb

Power element tube

Power element diaphragm (Power element charge) Evaporator pressure diaphragm Push rod Out

In Needle Seat

Spring

Adjustment screw

Figure 27. Dual-diaphragm TEV

PILOT-OPERATED THERMOSTATIC pressure is exerted on top of the main piston. A small EXPANSION VALVES bleed orifice in the top of the piston vents this pressure to the outlet side of the primary regulator. A The size of a conventional direct-operated thermosta- cage spring closes the primary regulator when the tic expansion valve, particularly the diaphragm size, is pressure supply to the top of the piston is cut off. generally adequate for its design system capacity. In large-tonnage systems, however, a direct-operated An increase in suction gas superheat reflects the valve would require a diaphragm or bellows operator need for increased refrigerant flow. In response, the of great proportions. It would not be economically or pilot-operated TEV moves in an opening direction. mechanically practical. For this reason, there are This action supplies greater pressure to the top of the pilot-operated valves for use on large direct expansion main piston. The piston moves to open the primary chillers and other large-tonnage applications. regulator, which provides increased refrigerant flow.

A typical application of a pilot-operated TEV is shown A reduction in suction gas superheat reflects the in Figure 28 on the next page. This assembly has a need for reduced refrigerant flow. In response, the primary regulator and a conventional direct-operated pilot valve moves in a closing direction. This action valve, which acts as a pilot. The pressure and tem- reduces the pressure on top of the piston. As the pis- perature of suction gas leaving the evaporator oper- ton moves to close the primary regulator, the volume ate the pilot valve—which, in turn, modulates the of refrigerant flow is reduced. In normal operation, primary regulator in response to changes in suction and in response to system load, the pilot valve and gas superheat. The primary regulator opens when regulator assume intermediate or throttling positions.

19 Remote External equalizer line Pilot thermal bulb well expansion valve

Pilot solenoid liquid valve

Shell-and-tube Pilot line strainer water cooler Suction line Liquid line

A Expansion valve Main line strainer Note: When suction line rises, dimension “A” should be as short as possible

Figure 28. Typical application of pilot-operated TEV

AMMONIA EXPANSION VALVES bulb to become chilled. The result is total or at least partial closing of the valve. Thermostatic expansion valves for ammonia refrigerant operate much the same as those used with other You might be led to think that valve response to refrigerants. Characteristics of liquid ammonia, how- changes in bulb temperature is almost instant, so ever, differ from those of fluorocarbon refrigerants. that superheat stays constant. This is not the case, Even small quantities of ammonia vapor have a great however. It takes time for refrigerant to travel from the erosive effect on the expansion device used. High- valve outlet through the evaporator to the remote velocity refrigerant and contaminants like dirt or scale bulb. There is some delay before the remote bulb compound this erosive effect. As with other parts of reacts to any change in valve seat position. Under ammonia systems, valves are made of steel and this condition, the valve “overshoots” on both opening steel alloys to combat the erosive effect. and closing. This condition is called hunting. With an exceptionally long evaporator, this may be very unde- EVAPORATOR LENGTH sirable. It could create alternate cycles, in which the evaporator would be overfed when there is not To simplify this chapter, several assumptions have enough superheat, and then starved when superheat been made up to this point. When a TEV opens, is too high. refrigerant almost immediately travels through the evaporator. There, it boils and superheats to a level There should be little hunting by a TEV with an evap- of, for example, 10°F.Then it proceeds to the remote orator that has short passes from the inlet to the out- bulb location on the suction line. let. Hunting will be most pronounced in evaporators with long tubes.Avoid this type of evaporator in the The subsequent increase in remote bulb temperature interest of efficient valve operation. If a large evapora- then develops pressure. This pressure bears against tor surface is required, it is better to use several short the diaphragm, opening the valve. Opening the valve tubes in parallel than one long tube. This reduces the allows more refrigerant to flow into the evaporator, travel time of refrigerant. If possible, the total length of decreasing the superheat and causing the remote evaporator tubing should not exceed 100 ft.

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